This detector supersedes both CircularFieldOfViewDetector and
DihedralFieldOfViewDetector, as it can handle any Field Of View shape.

This is work in progress! The detector is not tested at all.

This is related to issue #216.

The Eckstein-Hechler propagator now generated Cartesian orbits, with
velocity computed to be fully consistent with model evolution. A side
effect is that if users rebuild circular parameters from the generated
orbits, they will generally not math exactly the input circular
parameters (but position will match exactly).

Fixes issue #180.